In order to prevent corrosion of underground pipes, for example pipes carrying flammable gases, it is the practice to protect these pipes by cathodic protection. Cathodic protection is achieved and rust is prevented by making the potential of underground or underwater iron or steel structures or pipes approximately 0.3 volts more negative than its native electrochemical potential with respect to the surrounding soil. In such systems, a rectifier is often used, with its negative terminal connected to the structure to be protected and its positive terminal connected to a conductive anode or a series of parallel anodes buried in the earth some distance from the structure to be protected. In this connection it is normally desirable to locate the anodes about 100 feet or so from the pipe or other structure to be protected, but in many cases a lesser distance is used in view of geographical, boundary, or other constraints.
Typically one milliampere is needed per square foot of (bare) steel to be protected. This small amount of current per square foot is more or less independent of the resistivity of the soil surrounding the structure. Thus for practical purposes the resistance between a protected structure and the soil around it may be considered constant, and the number of amperes needed to protect a given structure is constant; and must normally be accepted as a system constraint.
Typically, sufficient rectifier voltage is used to create a flow of about one ampere per square foot of anode surface. The resistance between an anode and (remote) earth is variable, depending on the dimensions of the anode and linearly upon soil resistivity. Thus, an anode which has a resistance to earth of 2 ohms in soil having a bulk resistivity of 1,000 ohm-centimeters, will have a resistance of about 40 ohms in 20,000 ohm-centimeter soil. In accordance with the well-known expression relating voltage, current, and resistance, which indicates that voltage is equal to the product of current and resistance, when the anode has a resistance of 40 ohms, to obtain 4 amperes from the anode, a voltage of 160 volts would be required. The power at a single anode would then be excessive, and the anode would over-heat and fail quickly, as developed in my article entitled "Temperature Rise in Underground Impressed Current Anodes", Corrosion, Volume 36, No. 4, pages 161-167, April, 1980. To solve this problem, many anodes are normally installed in parallel when the soil resistivity is high, in order to lower the resistance, the anode temperature and the wasted power; and this practice which is generally followed is very costly. It is also noted that most of the resistance between an anode and the earth is concentrated in a shell of earth immediately surrounding the anode, with one-half of all resistance being found in a surrounding earth shell having twice the anode dimensions, and 90% being found in a shell having 10 times the anode dimensions.
In view of the fact that the anode area is usually small compared to the surface area of the metal to be cathodically protected, current density is relatively high near the anode; and because most of the anode-earth-cathode circuit resistance is in the soil immediately surrounding an anode, over 90% of the rectifier power output is typically lost in resistive heat losses in a shell of earth surrounding and close to the anode. This lost power which is proportional to the square of the current multiplied by the resistance, heats both the earth shell and the anode. The maximum current which can be safely delivered by an anode is limited by these heating losses and the thermal conductivity of the soil. In practice, corrosion engineers rarely attempt to increase soil thermal conductivity or to reduce soil resistivity to prevent overheating. Instead, they just install more anodes, with these anodes being connected in parallel and usually spaced 10 to 20 feet apart.
Attempts to deliver more amperes per anode through the use of higher voltage often result in anode failure. This may be caused by overheating, as the anode wire insulation is a thermoplastic and may be destroyed at relatively low temperatures, about 80 degrees C. to 105 degrees C., and it is also possible to boil away the water surrounding the anode.
Another cause of anode failure is the drying of the soil in the immediate area around an anode, even when the anode temperature is well below the boiling point of water. This phenomenon is known as electro-osmosis. Experiments show that with an anode and a cathode in a jar of clay mud, a direct current flow will rapidly cause drying of the mud near the anode and water saturation near the cathode. In actual practice in the field, when anodes are located in clay soil, they may carry 10 amperes for a day and then their resistance to earth will rise rapidly to 5 or 10 times its former value. This is apparently a result of this drying out phenomenon.
Summarizing, the cost of cathodic protection increases rapidly in high resistivity soils. More anodes are needed and/or more rectifier power output is needed. Raising the number of amperes per square foot of anode area often results in anode dry-out or burn-up. In practice, conservative corrosion engineers keep anode current density low, and use large numbers of anodes, with the resultant greatly increased expense.
Accordingly, the principal object of the present invention is to reduce the resistance between anodes and the surrounding earth, with the result that current costs, and the number of anodes which must be employed, are significantly reduced.